USB: f_mass_storage: Add support for performance recording

This patch adds support for populating the read/write performance values
for debugging the Mass storage performance numbers at VFS level.
To check the read/write values, user needs to invoke below command
cat /sys/class/android_usb/f_mass_storage/lun/perf
To reset the read/write values to zero
echo 0 > /sys/class/android_usb/f_mass_storage/lun/perf

Change-Id: Ied51969ce144a2335d2140259e28e8acbda44635
Signed-off-by: Chiranjeevi Velempati <cvelempa@codeaurora.org>
Signed-off-by: Rajkumar Raghupathy <raghup@codeaurora.org>
Signed-off-by: Azhar Shaikh <azhars@codeaurora.org>